Such a reactor comprises the following known elements:
a core containing reactive fuel elements for fuelling a nuclear fission reaction which creates a neutron flux in the volume of the core, said reaction itself being engendered in each zone of said volume by the neutron flux existing in said zone, and generating nuclear power in the form of heat which varies with said flux;
a coolant circuit having one branch passing through said core to remove said power and to enable it to be used in a variable load outside the reactor;
measuring means for measuring, in particular, the temperatures of said coolant in order to provide firstly an overheating ratio representative of the ratio of a predetermined critical heat flux which would be severely damaging at whatever point it occurs in the core, to a real local heat flux at a hot point where said flux is at a maximum, and also to provide an average temperature of the coolant which should be close to a normal value in order to enable the station to operate optimally; and
control rods provided with displacement means and with control means for inserting and extracting said rods longitudinally into corresponding zones of the core thereby controlling the positions of said rods, the position of a rod being defined by its extraction percentage.
Each of said rods includes a neutron-absorbing material distributed along its length and conferring negative reactivity thereto which diminishes said neutron flux and said nuclear power in the corresponding zone of the core over the depth to which the rod is inserted in such a manner as to enable displacement of said rods to vary the nuclear power of the core at the expense of distortions which appear in the distribution of the neutron flux and which give rise to additional wear of the core.
One family of these control rods is constituted by fine control rods capable of being displaced during normal control operations over a great height in order to cause the nuclear power to follow large variations which occur relatively infrequently in said external load.
Another family of the control rods is constituted by regulator rods, said rod control means receiving the average temperature of the coolant and reducing or increasing the extraction percentage of the regulator rods depending on whether said temperature is respectively greater than or less than its said normal value, thereby constituting a regulation loop which automatically maintains said average temperature in the vicinity of said normal value by causing the nuclear power to follow small variations which frequently affect said external load and which would cause said temperature to vary if the nuclear power were constant, said control means limiting the displacement of said regulation rods in normal operation to within a regulation range which is defined between two high extraction percentages so as to limit wear on the core by limiting the amplitudes of the distortions which such rod displacements frequently impose on the neutron flux.
One known method of controlling such a reactor comprises not only the displacement of the fine control rods during said normal control operations in order to cause the nuclear power to follow major changes in the external load, but also possible additional insertion of said rods during an operation to reduce power on an alarm which occurs as soon as said overheating ratio falls below a low alarm threshold such as to raise said ratio at least as far as said threshold, said threshold being determined with a safety margin sufficiently greater than unity to ensure that on being reached, said overheating cannot be approached in a dangerous manner, even in the event that certain accidents studied in advance may occur;
In this known method, the low alarm threshold of the overheating ratio is set at 2.17. Unfortunately, relatively short and unavoidable fluctuations appear from time to time in the operation of a reactor. They oblige the station operator to reduce reactor power too often by inserting fine control rods to bring the ratio back over the threshold, even though the average temperature of the coolant would allow operation at higher power. As a result the power available for delivering to the external load is reduced. The load is generally a turbine alternator set feeding electricity distribution mains. Such power reductions give rise, in the end, to a substantial reduction in the availability of the power station for producing electricity.
A particular aim of the present invention is to reduce the number of power reductions that may be caused by fluctuations affecting the overheating ratio in a manner which is simple and cheap, and without running any risks of danger due to local overheating in the reactor core.
The invention also aims to increase the availability, the profitability and the average power of a nuclear power station.